1. Field of the Invention
The present invention relates to an electromagnetic microwave wave spatial filter with circular polarization. One particularly interesting use of such a filter is the construction of an antenna of the Cassegrain type with a primary source emitting circularly polarized waves.
2. Description of the Prior Art
A filter with circular polarization must by definition have the following properties: be transparent for the incident waves transmitted with a given circular polarization and be reflecting for incident waves transmitted with a reverse circular polarization.
According to a first embodiment existing at the present time, a circular polarization filter is formed of five networks of wires, shown in FIG. 1, the description and operating principle of which will be explained in what follows.
The filter comprises five networks 1 to 5, parallel to each other, centered on the same axis .DELTA. and formed of parallel conducting wires. The first two networks 1 and 2 as well as the last two 4 and 5 are separated by dielectric spacers 8 and 9 of a given thickness e.sub.1. These dielectric spacers serve both as support for the networks and as a path of given length for the waves transmitted between these networks. For these four networks 1, 2, 4 and 5, the pitch d.sub.1 is close to a quarter of a wave length at the central frequency of the operating band. With respect to a reference system defining the incidence angle and the polarization of the incident wave and formed by two orthogonal axes OX and OY whose origin O is situated on the axis .DELTA. and perpendicular to this latter, the wires 6 of these four networks are parallel to the same direction forming an angle of 45.degree. with the direction of the wires of network 3. These networks have a self-inductive reflection coefficient for the component of the field parallel to the wires: EQU r.sub.1 =1/2(j-1)
j being the imaginary number such that j.sup.2 =-1 and r.sub.1 =0 for the component of the field perpendicular to the wires.
In the particular case of the graphic representation of FIG. 1, the wires 6 are parallel to a direction forming an angle of 45.degree. with the axes OX and OY.
The third network 3 is parallel to the preceding ones and situated between the networks 2 and 4. The pitch d.sub.2 of the parallel metal wires 7 which form it and which are parallel to the direction of the reference axis OY is very much less than .lambda./4, so that this network 3 has a reflection coefficient r.sub.2 =-1 for the component of the field parallel to wires 7 and r.sub.2 =0 for the component of the field perpendicular to the wires.
The first assembly formed of the first two networks 1 and 2 separated by the dielectric web 8 play the role of a circular polarizer transforming the circular polarization of the incident waves into a rectilinear polarization. This rectilinear polarization is orientated at 45.degree. with respect to wires 6. The central network 3 with closely spaced pitch plays the role of rectilinear polarization filter, transparent for a rectilinear polarization perpendicular to the direction of wires 7 and totally reflecting for rectilinear polarization parallel to their direction. Finally, the second assembly formed by the last two networks 4 and 5 separated by the dielectric web 9 serves as polarizer, transforming the rectilinear polarization of the waves transmitted by the preceding filter into a circular polarization.
The operation of the circular polarization filter which has just been described is for example as follows.
There will be obtained, at the output of the first circular polarizer a rectilinear polarization with direction parallel to axis OY for an incident left hand circular polarization for example and a rectilinear polarization with direction parallel to axis OX for an incident right hand circular polarization. Then, the incident wave whose circular polarization direction is such that the first polarizer transforms it into a wave with rectilinear polarization with direction parallel to the wires 7 of the rectilinear polarization filter is totally reflected thereby and passes through the first polarizer in the reverse direction which retransforms it into a circular polarization wave of the same direction. On the other hand, the reverse circular polarization incident wave is transmitted by the filter with a rectilinear polarization whose direction is perpendicular to the wires 7 of the filter. It is finally transformed again by the second polarizer into a wave with right hand circular polarization.
Thus, depending on the orientation of the wires 7 of the central network 3 with closely spaced pitch, the overall filter is only transparent for a left or right hand circular polarization.
A second known construction of a circular polarization filter is described in the French patent filed on Dec. 30, 1966 in the name of the applicant and published under the number 1 512 598. This filter, shown in FIG. 2 is formed by a network 10 of resonating elements 11, each element being formed by a metal wire bent into three sections 12, 13, 14 perpendicular to each other, in the form of "cranks".
These two embodiments have the disadvantage of being difficult to construct, the first because of the number of elements to be formed and assembled leading to a heavy solution from the mechanical point of view, the second because of the three dimensional wire network.